Super sensitive to B.O.? Maybe blame your genes

Genes may partly determine how strongly you smell the noxious aroma of a stinky armpit, new research shows. 

Similarly, your genetics influence how you perceive the smell of galaxolide, a "woody" synthetic musk used in fragrances and cleaning products, according to a new study, published Thursday (Feb. 3) in the journal PLOS Genetics. The new research reveals that people perceive these familiar scents as more or less intense depending on which versions of specific genes they carry. These key genes code for odor receptors, the specialized proteins on sensory cells that detect odor compounds.

Subtle differences between your odor receptors and someone else's can result in drastic differences in how you each perceive various scents, said Joel Mainland, co-senior author of the study, an associate member of the Monell Chemical Senses Center in Philadelphia and an adjunct associate professor of neuroscience at the University of Pennsylvania's Perelman School of Medicine.

Related: Genetics by the numbers: 10 tantalizing tales

"You have about 400 [types of odor] receptors, but you smell many more than 400 odors," Mainland told Live Science. That's because, rather than each odor compound activating just one odor receptor, a single odor compound can activate a slew of receptors. So when you catch a whiff of something, a number of odor receptors light up in your nose. The brain then notes which combination of receptors is active and decodes this information to identify the delicate scent of a rose, or the pungent stench of B.O.              

Because our perception of smell starts at our receptors, "which set of receptors you have has an enormous impact on what you can smell, and so any variation in that set of receptors will change the way that you smell," Mainland said. 

In their new study, Mainland and his colleagues explored how people's genetics shape the odor receptors they carry and, therefore, dictate how different scents smell to them. Genes and odor receptors aren't the only factors that shape our perceptions of scents, but they both play very influential roles, Mainland noted.

Several of the paper's authors are based in China, including co-senior author Sijia Wang, a professor and group leader at the Chinese Academy of Sciences; these authors had already gathered genetic data from a large population of Han Chinese people, who belong to a major ethnic group in China. From this population, the researchers analyzed data from 1,000 individuals, ages 18 to 55. They also repeated the study in a smaller cohort of about 360 ethnically diverse individuals in New York City, to see if their results carried over to different populations.

The team analyzed the whole genome of each participant and had each person do a smell test in which they rated the intensity and pleasantness of different scents on a 100-point scale. These odors included the synthetic musk galaxolide, as well as a compound called trans-3-methyl-2-hexenoic acid (3M2H), which can best be described as the characteristic smell of a stinky armpit. 

Many other odor compounds also contribute to the overall stench of body odor — about 120 compounds in all, the authors noted in their report. But among these, 3M2H is considered an "impact odor," meaning that it carries the characteristic scent of B.O.

With both the genomic and odor perception data in hand, the team began hunting for patterns between the participants' genetics and their reported perceptions of scents. 

They identified gene variants, meaning spots in the genome that differed between people at the level of single base pairs, that were tied to differences in smell test data, said co-first author Marissa Kamarck, a doctoral candidate at the University of Pennsylvania and a researcher in Mainland's lab at Monell. (Base pairs make up the individual "rungs" in the twisted ladder of DNA.) 

These tests highlighted the genes for two odor receptors, called OR4D6 and OR51B2, as critical for the perception of galaxolide and 3M2H, respectively. Prior to this study, scientists didn't know which scent compounds activated these receptors, Mainland said. 

In the OR4D6 gene, the team spotted two intriguing single nucleotide polymorphisms (SNPs), which are single "letter" changes in the DNA. These two SNPs often appeared together, though not always. The people who carried either SNP or both SNPs rated the scent of galaxolide as significantly less intense than people who carried neither SNP.

"The newly discovered receptor for galaxolide (OR4D6) may be essential for the perception of the odor," Kamarck wrote in a tweet about the study. "Most people who had two copies of a genetic variant of the receptor" — one from each parent — "were no longer able to smell galaxolide!" 

In contrast, people who carried a specific SNP in the OR51B2 gene perceived armpit stink, namely 3M2H, as more intense than did people without that SNP, the team found. Nearly 60% of the sample population carried this more-sensitive version of the gene.

Looking forward, Kamarck said she's interested in investigating whether variants in the OR4D6 gene, for galaxolide, also affect people's perception of other musks. Musks come in five distinct structural classes, meaning the different classes look very different from one another at the molecular level. However, all five classes of musk smell similar — the scents are often described as "sweet, warm and powdery," the authors wrote in their report. The team wants to look into whether OR4D6 gene variants affect the perception of other musks in the same class as galaxolide, as well as the other four classes.

There's also the question of when these different gene variants first arose and what their appearance can tell us about the evolution of humans' sense of smell, Mainland said. Based on previous research and the new study, the team found hints that, in general, humans' and other primates' sense of smell has degraded over time, in part, due to mutations in key genes for smell perception. However, this does not necessarily hold true for all genes, such as OR51B2, so the team wants to investigate this in more detail.

Originally published on Live Science.